Inkjet printers have gained wide acceptance. These printers are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. Inkjet printers produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes "dot locations", "dot positions", or "pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as an inkjet firing chamber resistor, located opposite the nozzle so ink can collect between it and the nozzle. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements. When electric printing pulses heat the inkjet firing chamber resistor, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper. The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Color inkjet printers commonly employ a plurality of print cartridges, usually either two or four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge contains a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge usually contains black ink with the other cartridge being a tri-compartment cartridge containing the base color cyan, magenta and yellow inks. The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles.
Ink-jet inks are mostly available as dye-based compositions. Typically, these inks comprise one or more water-soluble dyes and an aqueous vehicle containing one or more water-miscible organic solvents. Additionally, these inks may further comprise additives to improve a given property, such as waterfastness, color bleed, and the like. Various patents have been issued dealing with dye-based color components; examples of such patents, which are assigned to the same assignee as the present application, include U.S. Pat. Nos. 5,091,005; 5,098,476; 5,100,470; 5,106,416; 5,108,503; 5,112,399; 5,116,409; 5,118,350; 5,133,803; 5,196,056; and 5,198,023. In addition to dye-based inks, a limited number of pigment-based inks are also available. Pigment-based inks typically comprise one or more pigmented color components dispersed in a vehicle with a dispersant. Pigment-based inks offer two very desirable properties: waterfastness and lightfastness. However, they have not found extensive use in ink-jet ink compositions partly due to their natural tendency to agglomerate in aqueous media as well as their lack of uniform size distribution. Moreover, inks employing pigments as colorants do not avoid the problem of color bleed similarly experienced with dye-based inks.
Color bleed is a significant problem that can occur when multi-color images are printed using thermal inkjet technology as described above. In general and for the purposes set forth herein, color bleed is a term used to describe the diffusion/mixture of at least two different colored ink regions into each other. Such diffusion/mixture normally occurs when the different colored regions are printed next to and in contact with each other (e.g. at their marginal edges). For example, if a region consisting of a first coloring agent (e.g. black) is printed directly adjacent to and against another region consisting of a second coloring agent (e.g. yellow), the first coloring agent will often diffuse or "bleed" into the second coloring agent, with the second coloring agent possibly bleeding into the first coloring agent. Accordingly, indistinct images with a poor degree of resolution are produced. An insufficient degree of resolution results from the production of jagged, nonlinear lines of demarcation between adjacent colored regions instead of sharp borders therebetween. This can create significant problems, especially when high volume printing systems are used to print complex, multi-color images.
In addition, color bleed problems in multi-ink systems are also caused by strong capillary forces generated in many commonly-used paper substrates. These capillary forces cause a "wicking" effect in which coloring agents are drawn into each other by capillary action through the fibers of the paper materials. This situation also results in a final printed image of poor quality and definition.
Prior solutions to bleed have largely involved the use of accelerated drying, the use of a separate fixer solution to pre-coat the paper, or the use of special paper. A known solution of the bleed problem is to accelerate the evaporating of the solvent by heating the print medium as it is being printed and/or circulating dry air over the freshly printed image; however excessive heating interferes with the proper adherence between the ink and the print medium, and may also cause the less densely inked areas to shrink and/or to become brittle and discolored. Moreover, heating systems add cost and complexity to the printer. Fixing solutions also add cost and additional liquid to be dispensed. Special paper limits the user to a small, select group of papers that are more expensive than plain paper.
Bleed control has also been accomplished in different ways by the printer's "print mode" techniques, whereby adjacent dots are placed on successive sweeps by the pen in specified patterns and with fixed time delays between printing adjacent dots. However, such solutions decrease the throughput of the printer. At a time when the printer industry is in a pursuit to increase the throughput of printers, such a solution is unsatisfactory.
Other proposed solutions to the problem of bleed involve changing the composition of a thermal ink-jet ink to reduce bleed. For example, surfactants have been used to increase the penetration rate of the ink into the paper by reducing the surface tension of the ink. By increasing the penetration rate of inks into the paper, bleed is reduced between adjacently printed inks. However, increasing the penetration rate in this fashion may also reduce edge acuity.
Another method of reducing bleed between ink-jet inks involves employing a precipitating agent in a first ink to induce the precipitation of the colorant in a second ink printed adjacent to and in contact with the first ink. For example, U.S. Pat. No. 5,198,023 (issued Mar. 30, 1993 and assigned to the same assignee as the present application) discloses incorporating multivalent cations such as calcium chloride and magnesium chloride into yellow cationic aqueous-based inks to prevent bleed between yellow and black inks by precipitating the carboxyl/carboxylate group associated with the black colorant.
Another method of reducing bleed between ink-jet inks involves the use of pH-sensitive dyes as disclosed in U.S. Pat. No. 5,181,045, entitled "Bleed Alleviation Using pH-Sensitive Dyes", issued Jan. 19, 1993 and assigned to the same assignee as the present application. It is disclosed therein that an ink having a pH sensitive dye would be prevented from bleeding into an adjacent ink having an appropriate pH. More particularly, the migration of the ink having the pH-sensitive dye is prevented by rendering the dye insoluble on the print medium by contact with the pH of the adjacent ink.
In addition to employing surfactants, precipitating agents, and pH-sensitive dyes to achieve bleed control, other available means of achieving bleed control between adjacently printed inks include (1) pairing an ink having a water-soluble colorant with an oil-based ink (see, e.g., U.S. Pat. No. 5,342,440, entitled "Black-to-Color Bleed Control in Thermal InkJet Printing); (2) pairing inks having colorants that, together, form an insoluble salt pair; and (3) pairing an ink having a pigment dispersed with a negatively charged dispersant with an ink having a vehicle containing a cationic charged polymer.
Notably, of all of the above-described methods for achieving bleed control by ink composition, only the method involving the addition of surfactants can be administered without also changing the composition or properties of a second ink. Therefore, most strategies to reduce bleed rely, to at least some extent, upon the addition of a surface active agent.
While the addition of a surface active agent may decrease edge acuity, the viscosity of the ink also plays an important, albeit lesser, role in the degree of edge of acuity exhibited by an ink. Viscosity controls the amount of ink dot spread, with a higher viscosity limiting dot spread. Accordingly, edge acuity is reduced for an ink that has a high lateral spread on the paper. Dot gain is defined herein as the amount of spread for a given dot size compared to no spreading. Thus, a low dot gain ink does not spread as much as a high dot gain ink. Another way of viewing this is that for a given dot size, where a smaller drop volume gives the same dot size as a larger volume of ink, gain is attained.
A high dot gain ink is advantageous in several ways. By employing a high dot gain ink, one may use a lower drop volume of ink. The amount of ink used to print a page may therefore be decreased for high dot gain ink, translating to a lower cost per page. Moreover, cockle of the paper may be reduced by employing a lower drop volume of ink because of the lesser amount of water contacting the paper. Therefore, a print cartridge with a high dot gain ink is advantageous in reducing cost-per-page, bleed, and paper cockle. However, the best edge acuity of a high dot gain ink is inferior to the best edge acuity of a low dot gain ink.
Essentially, the advantages of a high dot gain ink, namely reduced cost-per-page, bleed and paper cockle, must be balanced against the reduced edge acuity for such ink. When printing black text, edge acuity is an important feature. However, edge acuity is less important when printing color text because the contrast between the color print and a white paper is less than the contrast between black print and the white paper. Furthermore, reductions in bleed, paper cockle, and cost are more important when printing a color image because large amounts of ink are utilized. It is therefore advantageous for a printer to employ a print cartridge containing a low dot gain black ink and at least one other print cartridge containing a high dot gain color ink.
It will be appreciated by those skilled in this art that reducing the amount of ink used to jet a given droplet of ink while maintaining a high edge acuity for text printing requires balancing various considerations. Thus, an ink set that combines the advantages of all types of inks, both high dot gain and low dot gain as well as pigment-based and dye-based, while alleviating the disadvantages of each, is desired.
A need remains for an inkjet printer that employs an optimum blend of inks in an ink set, thereby producing high quality black text as well as high quality graphics images at a high through-put rate. The formation of high quality black text and color images onto a print medium requires a delicate balance of competing requirements. For example, a black ink should employ surface active agents in a sufficient concentration to avoid bleed, but must also minimize or eliminate the concentration of surface active agents in order to produce good text quality with good edge acuity. On the other hand, superior edge acuity may be sacrificed in color inks to achieve more important reductions in bleed, paper cockle, and cost-per-page.
The present invention represents a unique and highly effective approach in the control of color bleed in multi-color thermal inkjet printing systems. The methods described herein may be implemented at a minimal cost, and do not require the use of extra equipment, custom manufactured paper, and/or special paper coatings. The present invention therefore represents an advance in the art of thermal inkjet printing technology as described in greater detail below.